1. Field of the Invention
The present invention relates to a voltage detecting apparatus and especially to a voltage detecting apparatus detecting voltage of direct-current power source (here after, DC power source).
2. Description of the Related Art
Usually, as a voltage detecting apparatus detecting voltage of DC power source, there is an insulation detecting apparatus of Flying Capacitor method. For detecting an insulating condition of an insulation condition of the high-voltage DC power source, a value of earth-fault resistance is calculated based on a measured value of a voltage between both terminals of an ungrounded capacitor (i.e. flying capacitor) charged with a voltage of the high-voltage power source and a measured value of a voltage between both terminals of the capacity, one terminal of which is grounded through a resistor, similarly charged with the voltage of the high-voltage power source. Thereby, the insulation condition of the high-voltage power source can be detected (for example, see patent document 1).
The insulation detecting apparatus described in the patent document 1 charges a voltage to a capacitor, and detects a malfunction of a switch on a circuit for measuring a voltage between both terminals of the capacitor.
FIG. 5 is a circuit diagram of the insulation detecting apparatus described in the patent document 1. As shown in FIG. 5, the power source 3 is formed by connecting plural batteries from V1 to Vn in series. The power source 3 is insulated from a ground electric potential 7 of a low-voltage system, such as a microcomputer 11.
The insulation detecting apparatus of FIG. 5 has a bipolar type capacitor 9, a first switch S1 and a second switch S2. The first switch S1 connects a positive electrode of the power source 3 insulated from on the ground electric potential 7 to one terminal of the capacitor 9. The second switch S2 connects a negative electrode of the power source 3 to the other terminal of the capacitor 9.
A microcomputer 11 includes a voltage measuring function which converts a provided voltage into an input port A/D analog-to-digital and measures the voltage. The insulation detecting apparatus has a third switch S3 and a fourth switch S4. The third switch S3 connects the one terminal of the capacitor 9 to the input port A/D. The fourth switch S4 connects the other terminal of the capacitor 9 to the ground electric potential 7.
A first diode D1, a first resistor R1 and the capacitor 9 are connected in series from a point between the first switch S1 and the third switch S3 to a point between the second switch S2 and the fourth switch S4. A second diode D2 and a second resistor R2 are connected in series from a point between the first resistor R1 and the capacitor 9 to a point between the first switch S1 and the third switch S3. That is, the first diode D1/the first resistor R1 and the second diode D2/the second resistor R2 are connected in parallel.
A third resistor R3 is connected between a terminal, at side of the input port A/D, of the third switch S3 and the ground electric potential 7. A fourth resistor R4 is connected between a terminal, at side of the ground electric potential 7, of the fourth switch S4 and the ground electric potential 7.
The above mentioned first-fourth switches S1-S4 are applied with an optical MOSFET. These switches are insulated from the power source 3 and can be controlled by the microcomputer 11.
A fifth switch S5 is connected from a point between the second diode D2 and the second resistor R2 to the ground electric potential 7. The fifth switch S5 and a fifth resistor R5 are connected in series. The fifth resistor R5 is lower about resistance than the second resistor R2.
A malfunction detecting action of a switch of the insulation detecting apparatus having the above construction is explained. Regarding the first switch S1, in condition of making the other switches open, the second switch S2 is closed during a closed circuit time T1. When the first switch S1 is in an abnormal condition, for example, a closed circuit condition or a short circuit condition due to malfunction or etc, a circuit connecting the capacitor 9 to the power source 3 in series is formed. Thereby, during the closed circuit time T1, the capacitor 9 is charged and voltage VC between both terminals of the capacitor 9 increases. Therefore, when the third switch S3 and the fourth switch S4 are closed and the microcomputer 11 reads the voltage VC between both terminals of the capacitor 9 through A/D port (input port A/D), since the capacitor 9 is charged by abnormality of the first switch S1, the voltage VC between both terminals of the capacitor 9 is detected by the microcomputer 11. Since the voltage VC between both ends terminal of the capacitor 9 is detected, the microcomputer 11 judges that the first switch S1 is in abnormal condition.
Regarding the second switch S2, in condition of making the other switches open, the first switch S1 is closed during the closed circuit time T1. When the second switch S2 is in an abnormal condition, for example, a closed circuit condition or a short circuit condition due to malfunction, a circuit connecting the capacitor 9 to the power source 3 in series is formed. Thereby, during the closed circuit time T1, the capacitor 9 is charged and the voltage VC between both terminals of the capacitor 9 increases. Therefore, when the third switch S3 and the fourth switch S4 are closed and the microcomputer 11 reads the voltage VC between both terminals of the capacitor 9 through the A/D port, since the capacitor 9 is charged by abnormality of the first switch S1, the voltage VC between both terminals of the capacitor 9 is detected by the microcomputer 11. Since the voltage VC between both terminals of the capacitor 9 is detected, the microcomputer 11 judges that the second switch S2 is in the abnormal condition.
Regarding the third switch S3, in a condition of making the other switches open, the first switch S1 and the second switch S2 are closed during the closed circuit time T1. That is, a circuit connecting the capacitor 9 to the power source 3 in series is formed, and the capacitor 9 is charged during the closed circuit time T1. Thereby, the voltage VC of between both terminals of the capacitor 9 increases. After elapsing the closed circuit time T1, when furthermore a prescribed time tw1 shorter than closed circuit time T1 passes, in conditions that in conditions that shutting the third switch S3 off, the fourth switch S4 is closed for detecting an abnormality of the third switch S3. When the third switch S3 is in an abnormal condition, for example, a closed circuit condition or a short circuit condition due to malfunction, a circuit, which is connected with the microcomputer 11 detecting voltage VC between both ends terminal of the capacitor 9, is formed. Therewith, a discharge circuit from the capacitor 9 including the second resistor R2, the third resistor R3 and the fourth resistor R4 is formed. Therefore, when the microcomputer 11 reads the voltage VC between both terminals of the capacitor 9 through the A/D port, the A/D port of microcomputer 11 is applied the voltage VC between both terminals of the capacitor 9 by abnormality of the third switch S3. Thereby, the voltage VC between both terminals of the capacitor 9 is detected by the microcomputer 11. Since the voltage VC between both terminals of the capacitor 9 is detected, the microcomputer 11 judges that the third switch S3 is in the abnormal condition.
Regarding the fourth switch S4, in a condition of making other switches open, the first switch S1 and the second switch S2 are closed during the closed circuit time T1. That is, a circuit connecting the capacitor 9 to the power source 3 in series is formed. Thereby, the capacitor 9 is charged during the closed circuit time T1. After elapsing closed circuit time T1, furthermore after a prescribed time tw1 shorter than closed circuit time T1 passes, abnormality of the fourth switch S4 is detected. Thereby in condition of shutting the fourth switch S4 off, the third switch S3 is closed. When the fourth switch S4 is in an abnormal condition, for example, a closed circuit condition or a short circuit condition due to malfunction, a circuit, which is connected to the microcomputer 11 detecting the voltage VC between both terminals of the capacitor 9 is formed. Therewith, a discharge circuit from the capacitor 9 including the second resistor R2, the third resistor R3 and the fourth resistor R4 is formed. Therefore, when the microcomputer 11 reads the voltage VC between both terminals of the capacitor 9 through A/D port, the A/D port of the microcomputer 11 is applied the voltage VC between both terminals of the capacitor 9 by abnormality of the fourth switch S4. Thereby, the voltage of VC between both terminals of the capacitor 9 is detected by the microcomputer 11. Since the voltage VC between both terminals of the capacitor 9 is detected, the microcomputer 11 judges that the fourth switch S4 is in the abnormal condition.
After the above mentioned malfunction detecting action of each switch and a voltage detecting action for measuring insulation resistance, the fifth switch S5 is closed in condition that the third switch S3 and the fourth switch S4 are closed, and an electric charge of capacitor 9 is discharged through the second resistor R2.
For detecting a malfunction of the fifth switch S5, the first switch S1 and the second switch S2 are closed during closed circuit time T1. When the fifth switch S5 is in the closed circuit condition or the short circuit condition by malfunction, an electric charge, which is charged in the capacitor 9, is discharged through the fifth switch S5, thereby the voltage between both terminals of the capacitor 9 does not increase. When the detected voltage VC between both terminals of the capacitor 9 is equal to or lower than a prescribed voltage, the microcomputer 11 judges that the fifth switch S5 is in the abnormal condition.
Patent Document 1:    Japan published patent application 2004-170146